A blood test prescribed after persistent fatigue or unexplained weight loss: it is often in this very concrete context that the question of cancer through a blood test arises for the first time. Blood tests do not replace a biopsy or imaging, but they guide the diagnostic process and, recently, open the door to early detection methods for cancers that are otherwise difficult to spot.
Circulating tumor DNA: what the blood test really detects
When a tumor develops, it releases small fragments of DNA into the blood known as circulating tumor DNA (ctDNA). It is on this biological trace that the most recent tests rely. We are no longer just looking for an abnormal protein: we are directly tracking the genetic material of the tumor.
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In practice, cancer detection through blood tests via ctDNA works better for certain locations than for others. Brain tumors remain very difficult to access with this type of analysis because the blood-brain barrier limits the passage of tumor DNA into the bloodstream. For pancreatic cancer, on the other hand, where symptoms appear late and imaging lacks sensitivity at an early stage, the detection of ctDNA represents a real leverage.
One point to remember: the presence of ctDNA is not sufficient to make a diagnosis. It triggers additional examinations (imaging, biopsy). We are in the logic of an alert signal, not a verdict.
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MCED multicancer tests: where do we stand concretely
MCED tests (Multi-Cancer Early Detection) are among the most commented advancements. Their principle: analyze a single blood sample to detect signals associated with several dozen types of cancers simultaneously, and even indicate where in the body the tumor is located.
No country has integrated MCED tests into a mass screening program to date. Several large-scale clinical trials are underway, but validation remains gradual.
What the UK requires before generalizing
The UK national screening committee has set strict criteria to evaluate these tests. It requires a comprehensive portfolio of evidence:
- Randomized clinical trials demonstrating a benefit on mortality or morbidity, not just on detection.
- Implementation studies measuring the impact on the healthcare system (diagnostic cascades, burden on imaging and biopsy services).
- Economic modeling incorporating the cost of false positives and the psychological burden for patients.
- Research on access equity, to prevent a promising test from widening health inequalities.
This evaluation grid shows how the transition from the laboratory to primary care remains a long journey. A test can work technically without necessarily improving survival at the population level, especially if false positives generate unnecessary invasive examinations.
Classic tumor markers: useful but not for screening
Before MCED tests, blood tests in oncology primarily relied on tumor markers: PSA for the prostate, CA 125 for the ovary, ACE for digestive cancers, AFP for the liver. These proteins are measured daily in French laboratories.
Their main utility is not initial screening, contrary to what many patients believe. Tumor markers are mainly used for monitoring treatments and detecting recurrences. An increasing PSA level after a prostatectomy is a strong signal. An isolated PSA measurement in a symptom-free man can generate as much confusion as clarity, due to false positives related to benign causes (infection, prostate enlargement).
In practice, a panel of tumor markers is not prescribed “just in case.” The measurement is targeted, guided by a clinical context: symptom, family history, post-treatment follow-up.
Treatment monitoring via ctDNA: the most concrete advancement in the short term
While mass screening through blood tests is still under development, therapeutic monitoring via ctDNA is progressing rapidly. The PARADIGM study, published in Nature Cancer, showed that the persistence of detectable ctDNA between six and twelve weeks after the start of treatment is associated with a significantly worse prognosis at two years.
In practical terms, this means that a chemotherapy or immunotherapy protocol could be adapted much earlier than today, without waiting for the control scan at three or six months. For patients, this is a time-saving and potentially reduces the side effects of an ineffective treatment.
What hinders routine adoption
This approach is not yet used in everyday practice. Feedback on this point varies by center and cancer type. Several barriers persist:
- The cost of ctDNA analyses, significantly higher than a standard tumor marker measurement.
- The absence of standardized thresholds: each laboratory may use different technology with varying sensitivities.
- The lack of clinical trials demonstrating that modifying treatment based on ctDNA improves overall survival, not just an intermediate indicator.
Oncologists who use ctDNA often do so within research protocols or personalized care, not in primary care.
Blood tests and cancer: what to expect, what not to expect
The complete blood count (CBC), liver or kidney function tests, classic tumor markers: these analyses guide, alert, and monitor. They do not diagnose cancer on their own. MCED tests and ctDNA monitoring represent a paradigm shift, but their integration into standard care pathways will take several more years.
An abnormal blood test is not a cancer diagnosis, and a normal blood test does not rule out cancer. This dual reality remains the structural limit of any blood analysis in oncology. Diagnosis always relies on the convergence of biology, imaging, and pathology, with biopsy as the final reference.